- Generate keys with PuttyGen
2048 bit, SSH-2 RSA
add a key comment like name@machine
add a passphrase (this is optional and I haven't done it)
- Save the keys in a safe place
NOTE: You can also export the private key in other format (SSH, openSSH)
- Copy the public key on the server you want to login into
in the file ~/.ssh/authorized_key2
- MODIFY the access of ~/.ssh to be readable/writeable/executable ONLY by that user:
"chmod 0700 ~/.ssh"
- MODIFY the access of ~/.ssh/autorized_key2 to be readable/writeable ONLY by that user:
"chmod 0600 ~/.ssh/authorized_keys2"
- Disable the login with password by modifying (e.g. with vi) the file "/etc/ssh/sshd_config" as it follows:

Protocol 2 PasswordAuthentication no UsePAM no

NOTE: in /etc/ssh there is ssh_config and sshd_config choose the right one!